Bipolar junction transistors (BJTs) are commonly operated as switching element. Current flowing between the collector and emitter nodes of a BJT is switched by switching the current flowing through the base node of the transistor on and off, driving the transistor between saturated and cutoff states. The current capability in the "on," i.e., saturated, state is generally proportional to the area of the emitter. Accordingly, it generally is desirable to provide the largest possible emitter area in order to increase the saturation current capacity of the transistor.
FIGS. 1 and 2 illustrate a typical npn-type BJT according to the prior art. An N epitaxial layer 20 lies upon an N.sup.+ substrate 10, with a P base layer 30 lying on the N epitaxial layer 20. An N.sup.+ emitter region 40 is provided in the P base layer 30, on which an emitter terminal 50 is formed. A base terminal 52 is located on the on the base layer 30, spaced apart from the emitter terminal 50. A collector terminal 54 is formed on the substrate 10. For the on state illustrated in FIG. 1, the p-n junctions of the device are forward biased, i.e., the collector terminal 54 is positive with respect to the emitter terminal 50. The current at the periphery of the emitter region 40 typically is higher than the current at the center of the emitter region 40, as indicated by the broad and narrow arrows.
Turning the switching transistor off involves reverse biasing the p-n junction between the base layer 30 and the emitter region 40 to cutoff the current flowing between the collector terminal 54 and the emitter terminal 50. As the junction is reverse-biased, a depletion region in the base layer 30 shifts from just beneath the base terminal 52 towards the center of the emitter region 40, blocking current flow between the base terminal 52 and the collector terminal 54.
This shift is not instantaneous, however, and during the finite time it takes for the depletion layer to shift and block the current flow, current may become concentrated toward the center of the emitter region 40, as indicated by the broad arrow in FIG. 2. This current concentration phenomena tends to be exacerbated if the transistor is being used to switch an inductive load, as current from the inductive load tends to keep the emitter current constant as the conductive area of the emitter region 40 decreases. The current concentration generally causes heating in the central portion of the emitter region 40, which in turn tends to increase current density in the central portion of the emitter region 40 and cause even more heating. If the temperature rise induced by current crowding is high enough, it can permanently damage the transistor, a failure mechanism often referred to as reverse bias second breakdown. Reverse bias second breakdown may limit the envelope of voltage and current in which the transistor may be operated, referred to as the Safe Operating Area (SOA) of the transistor.
Current crowding and associated problems can increase as the area of the emitter region 40 increases, as increasing the area of the emitter region 40 tends to increase the time to required to reverse-bias the emitter-base junction. Thus, although increasing the area of the emitter region 40 can increase the current carrying capability of the transistor in the on state, it can also reduce the SOA of the transistor. Solutions to the current crowding associated with reverse bias second breakdown have been proposed in U.S. Pat. No. 4,388,634 to Amantea et al. (describing a BJT with a collector with profiled doping) and U.S. Pat. No. 4,416,708 to Abdoulin et al. (describing a bipolar transistor having an toothed emitter structure).